It is well known that a mobile communications system (MCS) offers various services such as cellular phone service, radio pager service and the like to subscribers connected to the MCS by radio. Basic components of the MCS include a first and a second electronic switching systems (ESS's), a plurality of transit networks (TN's), a base station system (BSS) and a mobile station (MS). The TN's are provided by TN companies and each TN contains one or more other ESS's to be used for relaying voice messages and/or data between the first and the second ESS's and trunks coupled between the other ESS's. Both or either one, e.g., the second ESS, of the first and the second ESS's is capable of handling voice messages and/or data transmitted from the subscribers connected thereto.
When a public switched telephone network (PSTN) subscriber of the first ESS lifts his telephone handset and inputs a dial number of, e.g., the MS subscriber, to make a call, the first ESS receives and processes digits corresponding to the dial number. In the MCS, the dial number is commonly comprised of a network identification (NID) code, a country code, an area code, a station code and a subscriber number, wherein the NID code is a service type code indicating one of various communications services such as cordless phone service, radio pager service, cellular phone service and the like.
Specifically, if the NID code in the dial number is received by the first ESS, it first translates the received NID code to identify the receiving subscriber, i.e., the MS subscriber, as either a PSTN subscriber or a mobile subscriber. And if the MS subscriber is identified as the mobile subscriber based on the translated result, the first ESS sends a routing information demand (RID) signal and subscriber identification (SID) information representing the MS subscriber to a home location register (HLR) connected between the two ESS's; and, if otherwise, it relays voice signals and/or data from the PSTN subscriber to the second ESS.
In response to the RID signal, the HLR retrieves TN selection data, predetermined by the MCS and stored in a memory of the HLR, to be used by the first ESS to choose one of the TN's. Further, in response to the SID information, the HLR retrieves roaming information related to the MS subscriber's current location from a set of roaming information which is predetermined by mobile subscribers of the second ESS and stored in the memory of the HLR. After retrieving both the TN selection data and the roaming information, the HLR formats them by using a conventional data format scheme to obtain and provide routing information to the first ESS.
The first ESS chooses one of the TN's in response to the TN selection data in the routing information so that the PSTN subscriber of the first ESS can communicate with the MS subscriber of the second ESS through the selected TN based on the roaming information in the routing information. For interfacing the second ESS with the MS, the BSS is connected therebetween, wherein the BSS contains a base transceiver station and a base station controller and there are used a radio, an A and an A-bis interfaces issued at Open Systems Interconnection (OSI).
However, in the conventional routing information generation method provided in the MCS, the TN selection data employed to select one of the TN's is determined by the MCS only, resulting in a communications path which is not fully optimized for the PSTN and MS subscribers.